Use of lead free-alloys for C4 balls is increasing to reduce environmental impacts. Upon solidification, the tin-based (Sn-based) prior art alloy materials tend to form large grains in the lead-free C4 balls. The number of grains in the solidified lead-free C4 balls is correspondingly small, e.g., typically from 1 to 5, and more typically from 1 to 3. Tin has a tetragonal structure and the physical properties, such as deformation under stress and diffusion, depend upon Sn-grain orientation. When a small number of Sn-grains are present in a typical C4 ball, undesirable Sn-grain orientation will result in stress buildup in Si-device and decrease in electromigration reliability.
Thus, formation of a small number of grains in the lead-free C4 ball, e.g., 1 or 2, creates more stress on the semiconductor chip than formation of a large number of grains in the lead-free C4 ball, all other parameters for the C4 balls being equal. Many-grained C4 ball containing, for example, 5 or 10 grains per C4 ball with random orientation, is conducive to stress-mitigation and enhanced electromigration resistance. However, increasing the number of grains in a C4 ball has been very difficult to achieve. For electromigration reliability, decreasing diffusion rates inside the Sn-grains and along grain boundaries is desirable and may be achieved with suitable Sn-alloy compositions.